Hypertrophic cardiomyopathy (HCM) is characterized by enlargement of the ventricular muscle without dilation and is often associated with dominant pathogenic variants in cardiac sarcomeric protein genes. Here, we report a family with two infants diagnosed with infantile-onset HCM and mitral valve dysplasia that led to death before one year of age. Using exome sequencing, we discovered that one of the affected children had a homozygous frameshift variant in Myosin light chain 2 (MYL2:NM\_000432.3:c.431\_432delCT: p.Pro144Argfs*57;MYL2-fs), which alters the last 20 amino acids of the protein and is predicted to impact the C-terminal EF-hand (CEF) domain. The parents are unaffected heterozygous carriers of the variant and the variant is absent in control cohorts from gnomAD. The absence of the phenotype in carriers and infantile presentation of severe HCM is in contrast to HCM associated with dominant MYL2 variants. Immunohistochemical analysis of the ventricular muscle of the deceased patient with the MYL2-fs variant showed marked reduction of MYL2 expression compared to an unaffected control. In vitro overexpression studies further indicate that the MYL2-fs variant is actively degraded. In contrast, an HCM-associated missense variant (MYL2:p.Gly162Arg) and three other MYL2 stopgain variants that lead to loss of the CEF domain are stably expressed. However, stopgain variants show impaired localization suggesting a functional role for the CEF domain. The degradation of the MYL2-fs can be rescued by inhibiting the cell's proteasome function supporting a post-translational effect of the variant. In vivo rescue experiments with a Drosophila MYL2 -homolog ( Mlc2 ) knockdown model indicate that neither MYL2-fs nor MYL2:p.Gly162Arg supports regular cardiac function. The tools that we have generated provide a rapid screening platform for functional assessment of variants of unknown significance in MYL2 . Our study supports an autosomal recessive model of inheritance for MYL2 loss-of-function variants and highlights the variant-specific molecular differences found in MYL2 -associated cardiomyopathies.

Novel frameshift variant in MYL2 reveals molecular differences between dominant and recessive forms of hypertrophic cardiomyopathy